There are four main methods of heat exchange. These are radiation, conduction, convection, and evaporation. Evaporation is a very efficient cooling method. Convection is the heat transfer associated with the movement of mass across a thermal boundary. More specific, it relates to the flow of a fluid (coolant) past a solid boundary. Natural convection is convection caused by the motion and mixing caused by density variations within the fluid. Forced convection is the term used when this flow is caused by an outside force, such as a pump. The factors that affect convection efficiency include fluid velocity, fluid viscosity, and fluid heat capacity.